Performance Comparison of Optimum and MMSE Receivers for High Speed Wireless OFCDM Systems

Performances of optimum and minimum mean square error (MMSE) receivers for variable spreading factor orthogonal frequency and code division multiplexing (VSF-OFCDM) systems are compared in this paper. In VSF-OFCDM systems, the existence of multicode interference (MCI) in the frequency domain due to frequency-selective fading channels dramatically degrades the system performance. A quasi-analytic bit error rate performance is presented in the presence of imperfect channel estimations. Numerical results show that with linear computation complexity the MMSE receiver can improve system performance significantly by suppressing the MCI, although it cannot perform as well as the optimum receiver. Thus, with a small number of code channels, an optimum receiver can be employed to achieve better performance, whereas the MMSE receiver is suitable for a system with a large number of code channels due to simple complexity. In addition, the MMSE receiver is more robust than the optimum receiver to the different configurations of system parameters, e.g., spreading factors. Moreover, it is found that pilot channel power should be carefully assigned by making tradeoffs between the channel estimation quality and the received SNR for each code data channel.